FIG. 1 schematically shows an in-wheel motor type electric vehicle. The electric vehicle shown has two motor drive assemblies A each including an electric motor as a driving source and mounted, respectively, in the pair of right and left rear wheels 1 as drive wheels such that the rear wheels 1 can be individually driven by the motor drive assemblies A.
Motor drive assemblies A used in such in-wheel motor type electric vehicles include an electric motor, a reduction mechanism for reducing the rotation of the rotor shaft of the electric motor, and a drive axle driven from the reduction mechanism. The rotation of the drive axle is transmitted to one of the rear wheels through a hub ring supported by the drive axle.
Since such a motor drive assembly is mounted in a rear wheel, its axial length has to be sufficiently short. In order to minimize the axial length of such a motor drive assembly, JP Patent Publication 2001-173762A proposes to mount the reduction mechanism inside the rotor of the electric motor.
If a planetary gear mechanism is used as the reduction mechanism, it is necessary to lubricate the reduction mechanism with oil in order to prevent deterioration in durability due to wear of gears and to minimize noise when gears mesh. It is also necessary to lubricate bearings rotatably supporting the rotor shaft with oil in order to prevent seizure of these bearings.
For this reason, in the motor drive assembly disclosed in JP Patent Publication 2001-173762A, oil is stored in the motor case in which the electric motor is mounted to cool the electric motor with the oil and also lubricate the planetary gear type reduction mechanism with the oil. An oil reservoir is provided on the wall of the chamber in which the electric motor is mounted to store oil splashed up by the rotor of the electric motor in the oil reservoir. Oil stored in the oil reservoir is used to lubricate a pair of bearings rotatably supporting the rotor shaft.
In the motor drive assembly disclosed in JP Patent Publication 2001-173762A, since the same oil stored in the motor case is used both to cool the electric motor and to lubricate the reduction mechanism, it is necessary to use oil in such an amount that the lower part of the outer periphery of the rotor is submerged in oil so that oil can be splashed up by the rotor. Since it is necessary to use oil having a high viscosity in order to lubricate the reduction mechanism, the efficiency of the electric motor may deteriorate due to the viscous resistance of oil.
Also, since metal dust generated due to wear of gears of the reduction mechanism is mixed into oil which is used to cool the electric motor, metal dust may enter the electric motor, thus detrimentally affecting the motor.